A ribbon microphone includes main components, i.e., magnets generating a magnetic field and a ribbon diaphragm vibrating in the magnetic field in response to sound waves. The magnets are disposed on both sides of the ribbon diaphragm in the width direction and generates a magnetic field between the magnets on both sides. The magnets are supported by a frame composed of a magnetic material. The ribbon diaphragm with appropriate tension has both ends in the length direction fixed to the frame. The ribbon diaphragm vibrates in the magnetic field in response to sound waves to generate electrical signals in synchronization with the sound waves. Ribbon diaphragms are generally composed of an aluminum foil having high conductivity and low specific gravity. Ribbon microphones are disclosed in, for example, Japanese Unexamined Patent Application Publication Nos. 2009-200764 and 2011-160080.
A ribbon microphone outputs signals proportional to the particle velocity of sound waves, i.e., the velocity of air molecules moving in the anteroposterior directions due to sound pressure. This configuration readily provides a bidirectional microphone in general use. A unidirectional microphone is preferred in use for collection and amplification of musical sound because of its ease of use. A unidirectional ribbon microphone is therefore favorable for these uses.
An originally bidirectional ribbon microphone can be changed to unidirectivity by combining the omnidirectional component with the bidirectional component of the ribbon microphone. The omnidirectional component driving the diaphragm can be acquired by the sound pressure difference between an air chamber and the exterior that are separated with the diaphragm that blocks sound waves toward the air chamber. The rear surface of the diaphragm may therefore face the enclosed air chamber.
In order to combine the omnidirectional component with the bidirectional component, an acoustic resistance is disposed at the entrance of the small air chamber adjacent to the rear surface of the diaphragm, and the omnidirectional component is applied to then acquire a velocity component from a rear acoustic terminal. Sound waves entering from the rear acoustic terminal are divided by the acoustic mass of the rear acoustic terminal and the acoustic resistance at the entrance of the air chamber, and are guided to the rear of the diaphragm. The air chamber is necessary as the acoustic resistance. In a ribbon microphone, an acoustic tube is adjacent to the rear surface of a diaphragm and functions as the air chamber.
A ribbon diaphragm used for a ribbon microphone however has low mechanical impedance due to its small mass in comparison with a diaphragm of a dynamic microphone. As a result, a ribbon microphone needs an acoustic resistance operated effectively down to a low frequency region and therefore includes an acoustic tube filled with an acoustic resistance composed of, for example, cotton on the rear side of the diaphragm.
An example acoustic tube used for a ribbon microphone as mentioned above is described in Akio Mizoguchi, Acoustical Science and Technology, Vol. 18, No. 5, pp. 275-285. One end of the acoustic tube has a rectangular opening into which one end of the ribbon microphone unit can be attached. The size of the rectangular open end of the acoustic tube is matched with the size of the ribbon microphone having, for example, a width of 2 mm and a length of 20 mm. The acoustic tube has a diameter gradually decreasing from the opened end along the length and a cross section changing into a round shape, and is, for example, a considerably long tube having an inner diameter of about 7 mm. The acoustic tube is filled with an acoustic resistor composed of, for example, cotton. The rear end of the acoustic tube is closed while the rear space of the diaphragm is enclosed to provide an omnidirectional component. A part of the acoustic tube is provided with a hole. An opening or closing level of the hole can be adjusted with a shutter to adjust the directivity.
The typical size of the ribbon microphone is relatively small. Since a large area of the ribbon diaphragm is necessary for enhanced sensitivity, a ribbon microphone unit should also be large. A larger ribbon microphone unit however leads to a larger (wider and longer) acoustic tube. In order to attach the opened end of the acoustic tube to a microphone unit including a ribbon diaphragm having, for example, a width of 5 mm and a length of 50 mm, the opened end should have a size sufficient to cover the diaphragm. The other end of the acoustic tube should have an inner diameter of about 17.8 mm. Assuming that the entire length of the acoustic tube does not vary, the size of the ribbon diaphragm increases from 2 mm by 20 mm to 5 mm by 50 mm as described above to need a large acoustic tube having a volume of approximately 6.25 times that of the original tube.
A larger acoustic tube used for a unidirectional ribbon microphone leads to an increase in volume of the entire microphone and in length of the acoustic tube, which requires an appropriate design of the acoustic tube. In an example acoustic tube described in Akio Mizoguchi, Acoustical Science and Technology, Vol. 18, No. 5, pp. 275-285, a cylindrical member having a predetermined length is provided with multiple through-holes in parallel along its length, the ends of the through-holes being sequentially connected in series. The end of the acoustic tube provided by serial connection of the multiple through-holes is attached to one surface of the ribbon microphone unit.
As described above, a typical unidirectional ribbon microphone is provided with an acoustic tube. Use of an acoustic tube however increases the size of the entire ribbon microphone. A decrease in size leads to a small ribbon diaphragm, which cannot provide high sensitivity.
A small high-sensitivity unidirectional ribbon microphone could be achieved if an omnidirectional component could be extracted from the directional component of a ribbon microphone without an acoustic tube.